Monitor xfmr

[Multi-Volti Devices]

Why do xfmrs die?

Well, I originally figured it was ultra small wire (with a current rating of
1 mA, I imagined it had to be). I thought about it some, and realized the
current rating of the wire has to be much higher than that, or it would be
too small to handle. Geez, cells in your body don't vaporize 1 mA, why
should copper?)

Later (historically and education-wise), I figured it was that the wire had
been handled without gloves and corrosive hand perspiration ate thru the
wire. This is a known failure mode for ultra thin wire.

Later yet, working at Heath, the opinion among the techs was that capacitors
became leaky and drew too much current, causing the wire to open like a
fuse. I lived with this opinion for a while.

Much later (after reading transformer design manuals and MIL-207-x), I
learned that all components have a lifespan inversely related to
temperature.  I began to feel that the wir ewas probably not opening due to
excessive load current, because it would probably take 100 or more times the
rated secondary current of the hv winding (1 mA) to 'fuse'.

Insulation systems have finite lifespans. There are life curves for
different temperature class insulations at different temperatures. With this
sobering reality, I realized that a Heath monitor scope that lasted 25 years
was probably pretty well designed, after all. It's true those transformers
are more prone to failure than other products, and maybe they only last 10
years on average, not 25 (I threw out an arbitrary number).

A short circuit due to a problem o nthe secondary circuit could do it.

Corona is another problem in hv windings. Consistent impregnation of
windings is a challenge, and an air bubble or deterioration of the magnet
wire at elevated temperature can be a weak link where corona can develop,
over a period of time, and deteriorate to the point where an interturn or
interlayer short develops. This could lead to 'fusing' or openig of a
winding, or in somecases, hot spots which lead to further deterioration
elsewhere, and a heavier winding could short. So, some of you end up with
open secondaries, and some wind up (no pun intended) with shorted primaries
(the sadder and less salvageable of the two).

Murray

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